Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
Expert Tips and Hints
Off-Road Performance – Tyres & Suspension
Off Road Performance There are 2 Main Chassis Systems That Affect a 4x4 Off Road Performance
1. Tyres 2. Suspension
Tyres What’s important
Diameter
Rolling resistance
Ability to climb obstacles Contact patch length
Profile / Sidewall height
Resistance to rim damage
Contact patch length at lower
pressures
Pressure
Width
Tread Pattern
Grip ‘Digging’
Sinkage
Rolling resistance
Sinkage
Rolling resistance
Tyres Diameter If tyre pressure, ground strength and width are equivalent, then contact patch length must be equal
For equivalent contact patch lengths, there is more sinkage on the smaller wheel,
therefore it has more rolling resistance
Ground surface
Tyres Diameter
If TAN ∂ is greater than the contact patch friction
level then the wheel will not be able to generate
sufficient self traction to climb over the
obstacle and so will slip
If there is insufficient self-traction, then excess
traction from the other wheels is required to help
push the wheel onto the obstacle to generate more
traction, and help push the wheel over the obstacle
The larger ‘h’ is the more effective
any pushing force is at pushing the wheel
over the obstacle
∂
∂
h
h
Tyres
Profile / sidewall height
Higher sidewall tyre has more capacity to squash at lower tyre
pressures so contact patch length increase is greater
Greater chance of rim stone / rock damage during dynamic
tyre squash with smaller sidewall
Tyres Width
Wider tyre spreads the load better so
reduces sinkage
Despite less sinkage on the wider tyre,
longer
contact patch front may increase rolling
resistance
Equal contact patch areas
Longer contact
patch front
Tyres Tread Pattern
Aggressive tread pattern engages with the
ground so grip is a function of ground
strength not tyre to ground friction
If the ground has a low strength (sand)
then the tyre can dig in if it slips
Tyres Pressure
Reducing the tyre pressure lets the tyre
- squash
- increases the contact patch area
- reduces sinkage may reduce rolling
resistance
Increased contact
patch area
No increase in
contact patch
front length
Suspension Articulation – What is articulation
The ability of adjacent wheels to move in opposite directions to maintain ground
contact
Beam Axle Independent Axle
Articulation > Parallel Wheel Travel Articulation = Parallel Wheel Travel
Wheel Travel
Suspension Travel
Wheel Travel
Suspension Travel
Suspension Factors affecting articulation - Suspension type
Suspension Factors affecting articulation - Suspension type
Beam Axle Independent Axle
C of G C of G
Roll Centre
Roll Centre
Roll Moment Arm Roll Moment Arm
Longer Roll Moment Arm
More Roll Stiffness Required To control Roll Therefore
Greater Articulation Stiffness
Suspension Ground Clearance – Lateral profile
1. Discrete features - boulders / tree stumps Two cases to consider
2. Linear features - ruts
Discrete feature
The Vehicle can be manoeuvred to take full
advantage of the vehicle’s ground clearance
Linear feature
Little or no ability to
manoeuvre the vehicle
Particularly advantageous
if the differential is off-set -
Suspension Ground Clearance – Suspension type Change in ground clearance with suspension deflection
Beam axle Independent suspension
Ground clearance changes
as suspension moves up
and down
Suspension Ground Clearance – Suspension type
Change in wheel load with suspension deflection
Beam axle Independent suspension
Wheels Lift When Grounding Occurs Traction is Lost
Wheel Loads Reduce When Grounding Occurs Traction is Reduced
Suspension Ground Clearance – Suspension type
Ground clearance whilst articulated
Beam Axle - Ground clearance is maintained during articulation
Independent suspension - Ground clearance reduced during articulation